1. Field of the Invention
The present invention relates to a focus optical system for optical discs or optical disc master exposure apparatuses to be used for mass production of optical discs.
2. Description of Related Art
In production of optical discs, a stamper is first prepared by using a glass master as an optical disc master, and then copied reproduction of optical discs is performed by injection molding with use of the prepared stamper. Also, by an optical disc master exposure apparatus, recording light is projected to a recording layer (resist film) provided on the optical disc master to make the recording layer exposed to light, by which information is recorded on the optical disc master.
In such an optical disc master exposure apparatus, a coherent beam, which is light emitted (radiated) from a light source, is subjected to modulation or polarization in response to information to be recorded, thereby forming recording light. The recording light is converged to the recording layer of the optical disc master via an objective lens to make the recording layer exposed to light. Thus, information is recorded on the optical disc master. Further, it is known that an astigmatic method is used in focus control of the optical disc master exposure apparatus, i.e., in focus control for performing a positional adjustment of a distance between the optical disc master and the objective lens so that a spot shape of recording light converged on the optical disc master becomes closer to a specified focused point shape.
Now a typical configuration of a focus optical system adopted in conventional optical disc master exposure apparatuses is described with reference to FIG. 7.
As shown in FIG. 7, in a conventional optical disc master exposure apparatus 100, a coherent laser beam L emitted from a light source 101 passes through a power-and-signal modulating section 102, a polarizer 103 and a beam shaping section 104, and is reflected by a mirror 105 and a half mirror 106, partly transmitted by a transmitting mirror 108 and reflected by a dichroic mirror 109, thus the laser beam L being directed toward an optical disc master 111. Further, passing through an objective lens 110a that has been adjusted in its distance to the optical disc master 111 by an actuator 110 serving as a lens drive unit, the laser beam L is converged to the optical disc master 111. As a result, in the optical disc master 111 in which a resist film 111a serving as a recording layer (organic or inorganic resist layer) is placed on a glass master 111b, the resist film 111a is exposed to the laser beam L, so that information is recorded thereon. Meanwhile, a reflected beam (reflected light) of the laser beam L from the optical disc master 111 travels in reverse directions, and is partly reflected by the transmitting mirror 108 so as to be diverged from an optical path of the laser beam L. Thereafter, the diverged reflected beam is reflected by a mirror 112, passing through a converging lens (convex lens) 113a and a cylindrical lens 113b so as to be converged to a 4-division photodetector 113c. 
The converging lens 113a and the cylindrical lens 113b cause an occurrence of astigmatism. The reflected beam is received by the 4-division photodetector 113c in different spot shapes depending on whether the optical disc master 111 is positioned closer to (i.e., a case where a distance between the objective lens 110a and the optical disc master 111 is shorter than the focal length), or coincident with, or farther from a focal point position of the objective lens 110a (i.e., a case where the distance between the objective lens 110a and the optical disc master 111 is longer than the focal length). More specifically, on condition that the optical disc master 111 is positioned closer to the focal point position of the objective lens 110a, a spot M of the reflected beam results in a laterally elongated elliptical shape on the 4-division photodetector 113c as shown in FIG. 8A. On condition that the optical disc master 111 is positioned at the focal point position, the spot M of the reflected beam results in a circular shape as shown in FIG. 8B. On condition that the optical disc master 111 is positioned farther from the focal point position of the objective lens 110a, the spot M of the reflected beam results in a longitudinally elongated elliptical shape as shown in FIG. 8C. It is noted that in the optical disc master exposure apparatus 100, the converging lens 113a, the cylindrical lens 113b and the 4-division photodetector 113c are referred to as an astigmatic optical system 113 (see FIG. 7).
FIG. 9 shows a configuration of the 4-division photodetector 113c. As shown in FIG. 9, the 4-division photodetector 113c includes four photodetectors A, B, C, D having a light-receiving surface orthogonal to an optical axis of the reflected beam in the astigmatic optical system 113. The individual photodetectors A-D are placed in adjacency via two dividing lines orthogonal to each other. Upon reception of reflected light on the 4-division photodetector 113c including such four photodetectors A-D as shown above, a difference between sum signals (A+C) and (B+D) of diagonal photodetectors placed at mutually opposite positions bordering each other on an intersection point of the two dividing lines, the difference being {(A+C)−(B+D)} outputted by adders 120 and a subtracter 121, serves as a focus error signal that is a signal corresponding to a positional relation between the objective lens 110a and the optical disc master 111. In order that the focus error signal normally keeps a constant value (or near a constant value), drive of the actuator 110 is controlled to adjust the position of the objective lens 110a, by which the focal point position of the objective lens 110a is held on the resist film 111a of the optical disc master 111.
With the focal point position of the objective lens 110a adjusted to a specified focused point state as shown above, the modulated laser beam L is projected onto the resist film 111a, by which uniform pits or grooves are formed on the resist film 111a so that information is recorded thereon.
In such a conventional optical disc master exposure apparatus, various optical techniques such as disclosed in Literatures 1 to 8 are used.    Literature 1: JP H11-175987 A    Literature 2: JP 2007-287232 A    Literature 3: JP 2001-216659 A    Literature 4: JP 2002-329338 A    Literature 5: JP 2001-108877 A    Literature 6: USP Application Publication. No. 2007/0242575    Literature 7: USP Application Publication. No. 2007/0081441    Literature 8: U.S. Pat. No. 6,809,829